Lighting and display markets are undergoing rapid revolution led by innovation in light emitting diodes (LED) technologies. LEDs are solid state electronic materials made up of either inorganic materials (see e.g., C. Feldmann, T. Justell, C. R. Ronda and P. J. Schmidt, Adv. Funct. Mater. Vol 13, No. 7, 2003) and semiconductors, or organic (see e.g., The Electronic and Optical Properties of Amorphous Organic Semiconductors, M. E. Baldo, Ph.D. Thesis, Princeton University, 2001), or polymeric (see e.g., Polymer OLED technologies, Presentations from Cambridge Display Technologies (CDT)-Sumitomo Chemical Co., T. Yamada, Y. Tsubata, C. Sekine, T. Ohnishi, 2008, and I. Bidd, 2010) chemical entities. Actual requirements for these materials depend on the nature of the end application (display vs. lighting), but overall they provide many key benefits over existing materials, such as, for example, long device lifetimes, higher energy efficiency, low voltage of operation, better color rendering, and/or faster switching times.
Organic semiconductors have many unique advantages over inorganic LED materials such as crystalline semiconductor materials. Organic semiconductors have high absorption coefficients in the visible range, which offers the possibility to prepare very thin and flexible devices. A large number of molecules emit red shifted to their absorption, which reduces reabsorption losses in organic light emitting diodes (OLEDs). Organic semiconductors also have low indices of refraction that can ease light extraction issues, which is a key problem of inorganic LED materials (see e.g., R. Meerheim, K. Walzer, G. He, M. Pfeiffer and K. Leo, Organic Optoelectronics and Photonics II, edited by Paul L. Heremans, Michelle Muccini, Eric A. Meulenkamp, Proc. Of SPIE Vol. 6192, 61920P, (2006)). The organic LED materials emit light based on the phenomenon of fluorescence and phosphorescence. The phosphorescent OLED materials may play a larger role in improving the efficiency of light extraction of these LED devices because it is understood that phosphorescence may increase the efficiency by a factor of 4 over that obtained from fluorescence (see e.g. M. A. Baldo, M. E. Thompson and S. R. Forrest, Pure Appl. Chem., Vol. 71, No. 11, pp. 2095-2106, 1999; S. Kappaun, C. Slugovc and E. J. W. List, Int. J. Mol. Sci, 9, 1527-1547 2008). The organo-transition metal complexes are generally preferred over purely organic emitter materials because they enable this enhancement of the efficiency of the light-emitting devices via phosphorescence (see e.g., H. Yersin and W. J. Finkenzeller, Highly Efficient OLEDs with Phosphorescent Materials. Edited by H. Yersin, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008). The organo-metallic complexes also offer benefit in terms of low power consumption.
Generation of white light is required for substitution of existing lighting infrastructure with the LEDs. In order to generate white light, multiple strategies are used such as (a) combining individual RGB color phosphors, or (b) using a blue or UV LED with multiple phosphors generating a broad spectrum of white light. The latter approach is considered a simpler and a cheaper way to build a LED for generating white light.